Determine the body width of the over-sized couch by placing the end of the measuring tape on the floor in front of the couch. Measure up the front of the couch, over the seat cushions, up the back cushion and stop at the floor behind the couch. Avoid pressing on the over-sized cushions to get the most accurate measurement. Increase the measurement by 6 inches.
Determine the body length of the over-sized couch by placing the measuring tape at one end of the couch. Take the measurement from one side to the opposite side. Increase the measurement by 2 inches.
Determine the side length of the over-sized couch by placing the measuring tape on the floor below the side of the arm. Measure over the top of the over-sized arm and stop at the cushion. Increase the measurement by 12 inches.
Determine the side width of the over-sized couch by placing the measuring tape at the front edge of the couch. Take the measurement from the front edge to the back edge. Increase the measurement by 2 inches.
Create two pattern pieces using the measurements. Steps 1 and 2 create a rectangle body piece and Steps 3 and 4 create a rectangle or square arm piece. Two arm pieces are needed to complete the cover.
Lay the body cover piece onto the over-sized couch. Drape the arm pieces onto the couch so they line up in the correct spot. Pin the edge of each arm piece to the body piece so the right sides are together. Sew the arm to the body with a 1/2-inch seam.
Place the T-shaped slipcover over the couch with the wrong side facing up. Make sure the arm pieces line up correctly. On each side of the cover, grab the front edge of the body and arm and pin the two pieces together. Repeat this on the back edges of the cover. Sew all four edges with a 1/2-inch seam to create a cover in the shape of the couch.
Turn 1/4 inch of fabric to the backside of the cover around the bottom edge of the cover and iron to hold in place. Turn under another 1/2 inch of fabric to hem the cover. Sew the hem close to the inside-turned edge.